Purification of naphthalene



May 27, 1 R. SCOTT ET AL PURIFICATION OF NAPHTHALENE Filed Aug. 2, 19486 Z a x m a w m 5 W (m MW w M ow Q W Q 1 r l l l L l l lH (A. HI I I I II l h h l H h O\ i rney Patented May 27, 1952 PURIFICATION OF NAPHTHALENE Roland Scott and Eric H. J oscelyne, Oldbury,

Birmingham, England, assignors to The Midland Tar Distillers Limited,flldbury, Birmingham, England, a British company Application August 2,1948, Serial No. 42,036 In Great Britain August 13, 1947 4 Claims.

This invention relates to the crystallisation of chemical compounds fromliquid form and to apparatus for use in such crystallisation.

An object of this invention is to provide an improved process forcrystallising chemical compounds from liquid form without allowing amass of crystals to form by caking.

Another object is to obtain technically pure crystals from impurematerials in an improved manner.

A further object is to improve the production of technically purenaphthalene from crude naphthalene.

Yet another object is to provide an improved combination of apparatusfor use in crystallising and purifying chemical compounds.

Broadly stated, in the process of the present invention the liquidcompound from which crystals are to be formed is introduced into astream of liquid in which the desired crystals are either insoluble orhave low solubility and which is at a temperature low enough to causecrystals to form; this liquid may conveniently be described as theprecipitating liquid. The crystals and the precipitating liquid flowtogether while the crystals grow and then the crystals are conveyedupwards out of the liquid and while being so conveyed are washed by adownwardly flowing stream of the same (or precipitating) liquid. Thisstream of liquid serves to wash the crystals, and on arriving at thepoint at which the crystals are taken out of the first stream it meetsand moves with the latter and the two streams are removed together as acommon stream.

The apparatus we use consists of the combination of a crystallisingapparatus and a washing apparatus. The crystallising apparatus comprisesa tube or trough containing a conveyor, and the washing apparatuscomprises an upwardly inclined conveyor screw or the equivalent. Thesetwo pieces of apparatus are united by a common chamber in which the twoliquid streams mix and which is furnished with an outlet for the removalof the mixture as a common stream.

The invention is particularly applicable to processes in which crudenaphthalene is simultaneously crystallised and purified by introductioninto a precipitating liquid which is a solvent for the impurities in thecrude naphthalene and which may advantageously be methyl alcohol, asdescribed in our application Serial No. 8,298, dated 13th February,1948, now abandoned. We have found that on introducing naphthalene at atemperature in the order of 100 G. into cold methyl alcohol at, say,room temperature, shock cooling takes place and many very small crystalsare produced. These crystals tend to become caked into" a solid mass andthis caking is prevented by means of the invention because the crystalsare immediately engaged by the conveyor in the crystallising apparatusand carried along and in contact with the methyl alcohol. While they areso carried they tend to grow, i. e. naphthalene not crystallised uponimmediate contact with the methyl alcohol crystallises out on to thecrystals already formed. This growth may advantageously be controlled byappropriate cooling of the crystallising apparatus, the temperaturebeing maintained at about 35 to C'. at the inlet and being reduced fromthe inlet to the outlet end, where it may be about 16 C. The growth ofthe crystals is advantageous because we have found that if they are tobesuccessfully washed clean of adherent impurities they should berelatively large, say, notless than two millimetres along their sides.Moreover, the crystals must grow to about this size if they are to becarried upwards by the conveyor screw of the washing apparatus insteadof slipping backwards along the screw. The higher the temperature of themethyl alcohol at the inlet end of the crystallising apparatus, thegreater is the crystal growth, and far more saisfactory results areobtained when this temperature is in the order of 35 to 40 C. than whenit is only 20 0. Moreover, the larger crystals emerge from the washingapparatus in a dryer state.

An apparatus suitable for use in such a process is shown in theaccompanying diagrammatic drawing. This apparatus consists essentiallyof a crystallising apparatus l and a washing apparatus 2 united by acommon chamber 3 The crystallising apparatus l consists of a tube 4 withan inlet 5 for crude naphthalene and an inlet 6 for methyl alcohol andits outlet end delivers into the chamber 3. A conveyor consisting of ashaft 1 with a helical blade 8 is mounted in bearings 9 and H1 at thetwo ends of the tube 4 and in operation is driven slowly, say at therate of ten revolutions per minute, by an electric motor II.

The washing apparatus 2 consists of a tube 12 in which a conveyor screw13 works, being driven by an electric motor [4. The tube l2 has an inletl5 through which a washing stream of methyl alcohol is introduced and atthe top an outlet I 6 through which the crystals conveyed upwards by thescrew l3 are discharged to fall under gravity into any suitablecontainer or on to a conveyor.

The chamber 3 has an outlet ll through which the two streams of methylalcohol which meet in the chamber are removed as a common stream. Theheight of this outlet determines the level of the liquid in the tube 4.

The tube 4 is surrounded by a jacket 18 into which water for coolingpurposes is introduced through an inlet l9, leaving by way of an outlet20.

It will be seen that the methyl alcohol is introduced into the washingapparatus at a point well below the upper end of the conveyor screw 13so that the latter carries the crystals out of the liquid and thus someat least of the liquid adherent to them drains away while they are stillin the washing apparatus.

If the draining is inadequate the crystals may be passed towards adrying chamber, being maintained at a temperature low enoughon the onehand to prevent any distillation or sublimation of the crystals, buthigh enough on the other hand to bring about evaporation of theentrained liquid. The conveying apparatus in the drying chamber maycomprise agitators which serve to turn the mass of crystals over so asto present as large an area as possible to the atmosphere inside thechamber.

In order to save fresh methyl alcohol, some of the impure stream removedthrough the outlet I! may be mixed with fresh methyl alcohol introducedinto the inlet 6.

It will be understood that in simultaneously crystallising and purifyingany other chemical a suitable precipitating liquid which is also asolvent for the impurities is chosen.

We claim:

1. A continuous process for purifying crude naphthalene comprisingcontinuously introducing crude molten naphthalene into a flowing streamof precipitating liquid in which naphthalene is of low solubility andwhich is also a solvent for the impurities and is at a temperature ofthe order of 20-40 C. to cause naphthalene crystals to form at once,causing the mixture to flow a sufficient distance to allow the crystalsto increase in size to not less than 2 millimeters and at the same timepreventing agglomeration of the crystals by agitating the flowingmixture, separating the crystals from said mixture by conveying saidcrystals upwardly out of said mixture, washing said crystals being soconveyed with a downwardly flowing stream of the same liquid, directingsaid downwardly flowing stream into the liquid stream from which saidcrystals are upwardly conveyed to mix therewith, and removing theresultant mixed stream.

2. A process as claimed in claim 1 in which the temperature of saidfirst mentioned mixture is reduced asthe flow proceeds.

3. A process as claimed in claim 1 in which the temperature of saidfirst mentioned mixture is maintained at about 35 to 40 C. at a point offormation thereof and progressively reduced to about 16 C. at the pointof separation of said crystals and said liquid.

4. A process as claimed in claim 1 and in which the precipitating liquidis methyl alcohol.

ROLAND SCOTT. ERIC H. JOSCELYNE.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 848,484 Newnham Mar. 26, 19071,081,949 duPont Dec. 23, 1913 1,810,217 Lohmann June 16, 1931 1,906,534Burke May 2, 1933 2,163,581 Boyd, Jr June 2'7, 1939 2,207,752 MillerJuly 16, 1940 FOREIGN PATENTS Number Country Date 260,129 Great BritainOct. 28, 1926

1. A CONTINUOUS PROCESS FOR PURIFYING CRUDE NAPHTHALENE COMPRISINGCONTINUOUSLY INTRODUCING CRUDE MOLTEN NAPHTHALENE INTO A FLOWING STREAMOF PRECIPITATING LIQUID IN WHICH NAPHTHALENE IS OF LOW SOLUBILITY ANDWHICH IS ALSO A SOLVENT FOR THE IMPURITIES AND IS AT A TEMPERATURE OFTHE ORDER OF 20-40* C. TO CAUSE NAPHTHALENE CRYSTALS TO FORM AT ONCE,CAUSING NAPHTHALENE FLOW A SUFFICIENT DISTANCE TO ALLOW THE CRYSTALS TOINCREASE IN SIZE TO NOT LESS THAN 2 MILLIMETERS AND AT THE SAME TIMEPREVENTING AGGOLMERATION OF THE CRYSTALS BY AGITATING THE FLOWINGMIXTURE, SEPARATING THE CRYSTALS FROM SAID MIXTURE BY CONVEYING SAIDCRYSTALS UPWARDLY OUT OF SAID MIXTURE, WASHING SAID CRYSTALS BEING SOCONVEYED WITH A DOWNWARDLY FLOWING STREAM OF THE SAME LIQUID, DIRECTINGSAID DOWNWARDLY FLOWING STREAM INTO THE LIQUID STREAM FROM WHICH SAIDCRYSTALS ARE UPWARDLY CONVEYED TO MIX THEREWITH, AND REMOVING THERESULTANT MIXED STREAM.